1. Field of the Invention
The present invention relates to a positive-working photosensitive composition that is used in a manufacturing process of semiconductors, such as ICs, a process of producing circuit boards for LCDs and thermal heads and other photofabrication processes, and to a pattern forming method using such a composition. More specifically, the invention is concerned with a positive-working photosensitive composition suitable for the cases where far ultraviolet light of wavelengths not longer than 250 nm is used as an exposure light source or a beam of electrons is used as an irradiation source, and with a pattern forming method using such a composition.
2. Description of the Related Art
Chemical amplification photosensitive compositions are materials that form patterns on substrates by undergoing irradiation with actinic rays, such as far ultraviolet rays, or radiation to generate acids in the irradiated areas, and causing reaction utilizing the acids as catalysts to make a difference in developer solubility between the unirradiated and irradiated areas.
When KrF excimer laser is an exposure light source, resins containing as their basic skeletons poly(hydroxystyrene)s showing weak absorption in the region of 248 nm are mostly used as the main components. Therefore, such photosensitive compositions have high sensitivity and can form good-quality patterns at high resolution, compared with traditional compositions containing naphthoquinonediazide/navolak resin combinations.
However, when light sources of shorter wavelengths, such as ArF excimer laser (193 nm), are used for exposure, even the chemical amplification photosensitive compositions mentioned above are not satisfactory, because compounds having aromatic groups intrinsically show strong absorption at wavelengths around 193 nm.
Accordingly, resists containing resins having alicyclic hydrocarbon structures have been developed with the aim of using ArF excimer laser as an irradiation light source. For instance, the resist composition containing an acid-decomposable resin having adamantane structures is disclosed in JP-A-9-73173. However, as finer patterning has been required, reduction in resist film thickness has become necessary, and resists have been required to have higher dry etching resistance. U.S. Patent 2005/0074690 discloses the resins containing repeating units having diadamantane structures.
Although the dry etching resistance correlates with the carbon density in resin and can be enhanced by increasing the carbon density, the resin becomes more hydrophobic as the carbon density is increased, sometimes resulting in aggravation of development defectiveness and deterioration in pattern formability. As to a solution to this problem, JP-A-2001-215704 discloses that the resins having specified high-polar polymerizing units in addition to the polymerizing units of adamantane structures are effective in improving wettability with an alkaline developer.
Even when such a measure was taken, there still occurred a development defectiveness problem that, when fine patterns 100 nm or below in line width were formed, lines of the patterns formed were linked together by micro-bridges although the resolution was excellent, so breaks of lines were caused in the subsequent etching process to lower yields of devices, and besides, it was not said that the performance on a mask error factor (referred to as “MEF” hereinafter) as one of image performances was sufficient. The term “MEF” signifies a phenomenon that variations in mask accuracy are amplified by resist. From the viewpoint of ensuring compatibility between those performances, not only the structures of specified repeating units but also the formula design including a combination with a photo-acid generator as a photosensitive component is of importance.